Conductive sheet material

ABSTRACT

A conductive microporous sheet material comprises primary carbon fibres having a cross-sectional dimension of at least 1 μm, secondary carbon fibres in the form of carbon nanofibres and a binding agent for binding said primary and secondary fibres. The material may be produced by a wet-laid non-woven (paper-making) process. The sheet material may be used as a gas diffusion layer for a fuel cell or an electrode material for a battery.

The present invention relates to a conductive microporous sheet materialfor use in electrical devices, particularly but not exclusively forbatteries and related devices.

There is an increasing demand for sheet materials which have amicroporous structure combined with electrical conductivity and a highlevel of chemical resistance. Such materials find application as gasdiffusion layers for fuel cells and as electrode materials forbatteries. The present invention seeks to provide materials which meetthese requirements as well as a method for the manufacture of suchmaterials.

According to the present invention there is provided a conductivemicroporous sheet material comprising primary carbon fibres having across-sectional dimension of at least 1 μm, secondary carbon fibres inthe form of carbon nanofibres and a binding agent for binding saidprimary and secondary fibres.

The sheet of the invention has a microporous structure determinedprimarily by the relative proportions of the first and second fibres.The variation in pore structure with the secondary (nanofibre) contentmay readily be determined experimentally by a person skilled in the art.Thus, for example, the sheet may be produced by a wet-laying technique(see below) and the experimental determination may be effected byproducing and testing laboratory produced single sheets (hard sheets).This information may then be used to select the appropriate blend ofprimary and secondary fibres for a given microporous structure.

The sheet of the invention may be a thin, flexible material.

The primary fibres preferably have a cross-section of 1 to 15 μm, morepreferably 4 to 12 μm, even more preferably 5 to 10 μm. Typically theprimary fibres will have a length of a few millimetres, e.g. 3 to 8 mm(about 6 mm). A preferred example of primary carbon fibre is SGL C25(available from Technical Fibre Products Ltd.).

The primary fibres may be obtained from acrylonitrile or pitch.

The secondary fibres (nanofibres) preferably have a cross section ofbetween 100 and 500 nanometres, more preferably between 100 and 250nanometres. The nanofibres may be produced by vapour deposition. Apreferred example of a carbon nanofibre is Pyrograf-III (available fromASI).

Preferably the primary carbon fibre constitutes between 10 and 90 wt %of the total weight of fibres and secondary fibres constitute between 10and 90 wt % on the same basis. Preferably the fibres together provide atleast 90% by weight of the sheet material.

The binding agent is required for adequate bonding strength of thematerial. The binding agent will generally constitute less than 10% byweight of the sheet material, and more typically less than 5% on thesame basis.

The binding agent may for example be a thermoplastic or thermosettingresin, a suitable example of which is a phenolic resin such as GP5520.Whilst the use of resin binding agent is perfectly satisfactory,improved conductivity can generally be achieved by use of carbon as thebinding agent. Sheets utilising carbon as the binding agent may beproduced by heat treatment in an inert atmosphere of a sheet materialincorporating a resin binding agent, said conversion of the resinbinding agent to carbon serving to increase conductivity whilstretaining the controlled microporous structure.

A sheet according to the present invention may have any one or anycombination of the following properties:—

-   -   A. A weight of between 10 and 200 g/m², more preferably about 50        g/m².    -   B. A thickness of between 0.1 and 2 mm, more preferably about        0.3 mm.    -   C. A Gurley air permeability of between 8 and 50 seconds/300        cm³.    -   D. A maximum pore size of less than 22 μm, more preferably less        than 16 μm and most preferably less than 12 μm.    -   E. A through plane resistance of less than 150 Ω/cm, more        preferably less than 50 Ω/cm.    -   F. A tensile strength of between 0.7 and 1.3 kN/m.

Conductive sheet material in accordance with the invention has a varietyof end uses, including:—

-   -   (1) Gas diffusion layers for fuel cells    -   (2) Electrode materials for batteries.

The conductive sheet material, according to the invention is preferablyproduced using a wet-laid non-woven (papermaking) process. The use of awet-laid production process allows a wide range of proportions of carbonfibres and carbon nanofibres to be used and thus lends itself toproduction of materials with highly specific pore structures.

The preferred method of manufacture is to form a slurry of the two fibretypes with binder by mixing the materials in water at a concentration ofup to 1% by weight (e.g. between 0.02 and 0.5 wt %). Mixing ispreferably carried out using a high speed agitator and the resultingslurry is formed into a suitable sheet material by passing through apapermaking former.

Fibre distribution and sheet forming may be aided by the use ofviscosity modifiers and/or drainage aids.

After forming liquid may be removed from the sheet by vacuum and/or hotair drying. Where both liquid removal methods are used it is preferredthat hot air drying is applied ultimately as it may be used to melt orcure the binder. It is preferred that the final stage of the productionprocess is the carbonisation of the binding agent.

Following the carbonisation stage the sheet material is preferablyformed into a continuous roll in order to facilitate further automatedprocessing.

Both continuous of batch processing of the sheet material are envisaged.

Preferably in the production process the binder is initially the form ofa powder although the use of a binder in any other physical form is notprecluded.

The invention will now be described further with reference to thefollowing non-limiting Examples.

EXAMPLE 1

A sheet was formed by mixing the following elements in water using ahigh speed agitator at a combined concentration of 0.5 wt %.

Carbon fibre (SGL C25), 6 mm chopped length 24 wt % Carbon nanofibre(Pyrograf-III, ex ASI) 73 wt % Phenolic resin (GP 5520)  2 wt %

The resulting material was converted into sheet form using a papermakingformer. The sheet was dried using a combination of vacuum and hot airand then carbonised by heating in an inert atmosphere until the phenolicbinder was completely converted to carbon.

Sheets formed from the above mixture had the following characteristics:—

Weight 50 g/m² Thickness 0.3 mm Tensile strength 0.7 kN/m Gurley airpermeability 50 seconds/300 cm³ Maximum pore size 12 μm Through planeresistance 150 Ω/cm

EXAMPLE 2

A sheet was formed by mixing the following elements using the sametechnique as in Example 1.

Carbon fibre (SGL C25), 6mm chopped length 49 wt % Carbon nanofibre(Pyrograf-III, ex ASI) 49 wt % Phenolic resin (GP 5520)  2 wt %Sheets formed from the above mixture had the following characteristics:—

Weight 50 g/m² Thickness 0.3 mm Tensile strength 1.0 kN/m Gurley airpermeability 20 seconds/300 cm³ Maximum pore size 16 μm Through planeresistance 150 Ω/cm

EXAMPLE 3

A sheet was formed by mixing the following elements using the sametechnique as in Example 1.

Carbon fibre (SGL C25), 6mm chopped length 74 wt % Carbon nanofibre(Pyrograf-III, ex ASI) 24 wt % Phenolic resin (GP 5520)  2 wt %Sheets formed from the above mixture had the following characteristics:—

Weight 50 g/m² Thickness 0.3 mm Tensile strength 1.3 kN/m Gurley airpermeability 8 seconds/300 cm³ Maximum pore size 22 μm Through planeresistance 150 Ω/cm

1. A conductive microporous sheet material comprising primary carbonfibers having a cross-sectional dimension of at least 1 μm, secondarycarbon fibers in the form of carbon nanofibers and a binding agent forbinding said primary and secondary fibers, wherein the binding agentconstitutes less than 10 wt % of the sheet material and the bindingagent is selected from the group comprising thermoplastic resins andthermosetting resins; and wherein the sheet has a tensile strength ofbetween 0.7 and 1.3 kN/m.
 2. A sheet material according to claim 1,wherein the secondary carbon fibers have a cross section of between 100and 500 nanometres.
 3. A sheet material according to claim 2, whereinthe secondary carbon fibers have a cross section of between 100 and 250nanometres.
 4. A sheet material according to claim 1, wherein theprimary carbon fibers constitute between 10 and 90 wt % of the totalweight of fibers.
 5. A sheet material according to claim 1, wherein thesecondary carbon fibers constitute between 10 and 90 wt % of the totalweight of fibers.
 6. A sheet material according to claim 1, wherein thebinding agent constitutes less than 5 wt % of the sheet material.
 7. Asheet material according to claim 1, wherein the binding agent is aphenolic binder.
 8. A sheet material according to claim 1, wherein thebinding agent is carbon.
 9. A sheet material according to claim 1,wherein the sheet has a weight of between 10 and 200 g/m².
 10. A sheetmaterial according to claim 9, wherein the sheet has a weight of about50 g/m².
 11. A sheet material according to claim 1, wherein the sheethas a thickness of between 0.1 and 2 mm.
 12. A sheet material accordingto claim 11, wherein the sheet has a thickness of about 0.3 mm. fibersin the form of carbon nanofibers and a binding agent for binding saidprimary and secondary fibers, wherein the binding agent constitutes lessthan 10 wt % of the sheet material and the binding agent is selectedfrom the group comprising thermoplastic resins and thermosetting resins;and wherein the sheet has a Gurley air permeability of between 8 and 50seconds/300 cm³.
 13. A sheet material according to claim 1, wherein thesheet has a maximum pore size of less than 22 μm.
 14. A sheet materialaccording to claim 13, wherein the sheet has a maximum pore size of lessthan 16 μm.
 15. A sheet material according to claim 14, wherein thesheet has a maximum pore size of less than 12 μm.
 16. A sheet materialaccording to claim 1, wherein the sheet has a through plane resistanceof less than 150 μ/cm.
 17. A sheet material according to claim 16,wherein the sheet has a through plane resistance of less than 50 μ/cm.18. A sheet material according to claim 1, wherein the primary carbonfibers have a cross section of between 4 and 12 μm.
 19. A sheet materialaccording to claim 18, wherein the primary carbon fibers have a crosssection of between 5 and 10 μm.
 20. A sheet material according to claim1, wherein the primary carbon fibers are between 3 and 8 mm in length.21. A sheet material according to claim 20, wherein the primary carbonfibers are about 6 mm in length.
 22. A sheet material according to claim1, wherein the sheet has a Gurley air permeability of between 8 and 50seconds/300 cm³.
 23. A conductive microporous sheet material comprisingprimary carbon fibers having a cross-sectional dimension of at least 1μm, secondary carbon.